A fluid, for example, a gas such as air may involve a problem in terms of noise. In particular, noise emitted from an axial blower, a gas turbine engine, etc. and noise leaking from an airflow passage of an apparatus requiring circulation of air for ventilation or waste heat, a soundproof chamber, or the like may be loud enough to constitute a problem. Such problem of noise is generally coped with, for example, by surrounding a noise generation source with a noise-proof cover, connecting a muffler to an air intake/exhaust port, or providing a sound absorbing material in the airflow passage. In the case as discussed here, however, a single pipeline serves as both the airflow passage and a noise propagation passage, and it is often rather difficult to prevent solely passage of the noise by a definite and effective means without causing any pressure loss in the airflow passage. In particular, to achieve a reduction in noise of low frequency, it is necessary to increase a thickness, volume of the sound absorbing material, or overall size of a structure of a silencer, so it is not easy to cope with the noise problem while taking into account the conveniences for practical use, such as compactness, lightness, and durability (see, for example, JP 2002-22254 A and JP 2005-31599 A).
The noise emitted from the axial blower, the gas turbine, etc. is generated as a dominant frequency sound of high sound pressure level, in which a sound of a frequency expressed by a product of the number of moving vanes of a fan and an RPM is acutely conspicuous. Such dominant frequency sound often has a much higher noise level than a level of noise generated from an airflow itself passing through the pipeline at high speed to constitute a predominant noise portion.